In a press conference this afternoon,
President Obama said he was "deeply concerned" by reports of
Russian troops mobilizing in Ukraine's Crimea region. Obama also
emphasized that "there will be costs" for any military intervention in Ukraine.

"It would present a profound
interference in matters that must be decided by the Ukrainian
people," he said, adding, "it would invite the condemnation of
the international community."

...

"The United States will stand with
the international community in affirming that there will be costs for
any military intervention in Ukraine."

Friday morning, dozens of armed men in military uniforms without insignias surrounded two Crimean…
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Reporters from the BBC also spotted
Russian transport vehicles entering the region and witnesses reported
seeing at least eight Russian military helicopters enter the region (unconfirmed video of which can be seen here). Reporters from the Global Post also filmed alleged Russian military convoys driving through Crimea.

Later in the day, a "Kiev official"
told the AFP and Sky News that more than 2,000 Russian troops landed
in Crimea as part of an "armed invasion." Ukraine's acting
president, Oleksandr Turchynov, released a statement, condemning the reported military intervention from Russia.

"In an act of naked aggression against
Ukraine, and under the pretense of military exercises, Russia has
brought military forces into the Autonomous Republic of Crimea. Not
only have they seized Crimea's parliament and Council, they've
tried to take control of civilian facilities and communications, and
tried to block the positions of Ukrainian forces.

"They are provoking us into an armed
conflict. Based on our intelligence, they're working on scenarios
analogous to Abkhazia, in which they provoke conflict, and then they
start to annex territory.

"Ukraine's military will fulfill
its duties, but will not succumb to provocation, and is not entering
into armed conflict, understanding the high danger that this would
expose to the civilian population of Crimea."